1. Field of the Invention
The present invention relates to a semiconductor device. Further, the present invention relates to a display device. Furthermore, the present invention relates to an electronic appliance including a display device in a display portion.
2. Description of the Related Art
Various metal oxides are used for a variety of applications. For example, indium oxide is a well-known material and used for a material of a transparent electrode which is needed in a liquid crystal display or the like.
Some metal oxides have semiconductor characteristics. As metal oxides exhibiting semiconductor characteristics, for example, tungsten oxide, tin oxide, indium oxide, zinc oxide, and the like can be given. References disclose a thin film transistor in which such a metal oxide exhibiting semiconductor characteristics is used for a channel formation region (Patent Documents 1 to 4, and Non-Patent Document 1).
Further, not only unitary oxides but also multiple oxides are known as metal oxides. For example, InGaO3(ZnO)m (m is a natural number) belonging to homologous series has been known as a multi-component oxide semiconductor including In, Ga, and Zn (Non-Patent Documents 2 to 4).
In addition, it has been confirmed that an oxide semiconductor including such an In—Ga—Zn-based oxide can be used as a channel layer of a thin film transistor (Patent Document 5, and Non-Patent Documents 5 and 6).
A TFT in which a channel formation layer is formed using an oxide semiconductor has a higher electric field mobility than a TFT using amorphous silicon.
TFTs which are formed using such an oxide semiconductor over a glass substrate, a plastic substrate, or the like are expected to be applied to display devices such as a liquid crystal display, an electroluminescence display (also referred to as an EL display), and electronic paper.
A semiconductor device such as a display device has a problem in that malfunction occurs or a circuit in a display device is damaged due to noise.
Examples of the noise include conductive noise, emission noise, and the like. Examples of the conductive noise include a high-speed burst wave and the like. Examples of the emission noise include electrostatic discharge and the like.
In order to reduce adverse effects of the noise, display devices provided with a variety of protective means against noise have been proposed (Patent Document 6).